DNase is a phosphodiesterase capable of hydrolyzing polydeoxyribonucleic acid. DNase has been purified from various species to various degrees. The complete amino acid sequence for a mammalian DNase was first made available in 1973. See e.g., Liao, et al., J. Biol. Chem. 248:1489 (1973).
DNase has a number of known utilities and has been used for therapeutic purposes. Its principal therapeutic use has been to reduce the viscoelasticity of pulmonary secretions in such diseases as pneumonia and cystic fibrosis, thereby aiding in the clearing of respiratory airways. See e.g., Lourenco, et al., Arch. Intern. Med. 142:2299 (1982); Shak, et al., Proc. Nat. Acad. Sci. 87:9188 (1990); Hubbard, et al., New Engl. J. Med. 326:812 (1992).
DNA encoding human DNase I has been isolated and sequenced and that DNA has been expressed in recombinant host cells, thereby enabling the production of human DNase in commercially useful quantities. See e.g., Shak, et al., Proc. Nat. Acad. Sci. 87:9188-9192 (1990). Recombinant human DNase (rhDNase) has been found to be useful clinically, especially in purified form such that the DNase is free from proteases and other proteins with which it is ordinarily associated in nature. See e.g., Hubbard, et al., New Engl. J. Med. 326:812 (1992).
The means and methods by which human DNase can be obtained in pharmaceutically effective form is described in the patent applications cited above. Various specific methods for the purification of DNase are known in the art. See e.g., Khouw, et al., U.S. Pat. No. 4,065,355 (issued 27 Dec. 1977); Markey, FEBS Letters 167:155 (1984); Nefsky, et al., Eur. J. Biochem. 179:215 (1989).
Although it was not appreciated at the time the above-referenced patent applications were filed, the DNase product obtained from cultures of recombinant host cells typically comprises a mixture of deamidated and non-deamidated forms of DNase. The existence of deamidated forms of DNase remained unappreciated notwithstanding that the phenomenon of deamidation of asparagine and glutamine residues in some proteins is known. See e.g., Eipper et al., Ann. Rev. Physiol. 50:333 (1988); Kossiakoff, Science 240:191 (1988); Bradbury et al., Trends in Biochem. Sci. 16:112 (1991); and Wright, Protein Engineering 4:283 (1991);
The present invention is predicated upon the previously unappreciated fact that recombinant human DNase may exist as a mixture of deamidated and non-deamidated forms. Using the methods of the present invention, it has been found that deamidated human DNase is less active enzymatically than non-deamidated human DNase. Thus, the presence of the deamidated DNase and non-deamidated DNase together in a mixture, and the potential for further deamidation occurring, such as has been found to occur upon in vitro storage of preparations of human DNase, may complicate efforts to provide consistent uniformity in a DNase product being administered clinically. Therefore, as the existence and characteristics of deamidated DNase were not known prior to the present invention, the methods for identifying deamidated DNase and separating it from preparations of DNase in which it may be found were unobvious at the time this invention was made.